I. Satellite Ground Station Requirements
The Fixed Satellite Service (FSS) downlink frequency band at 11.7 to 12.2 GHZ, commonly called the Ku-band, has been gaining wide acceptance as a reliable television transmission medium. One factor contributing to this has been higher power transponders enabling small aperture antennas to produce high quality television signals. The FCC has assigned the Ku-band primarily to satellite transmission, freeing it from terrestrial microwave interference. There have also been significant developments in semiconductor technology, design and manufacturing techniques which have resulted in the improved performance of earth station hardware while reducing installation and maintenance costs. Finally, after years of propagation studies and operational experience, the effects of precipitation and proper link design are now well understood.
When operating at frequencies above 10 GHZ, which includes the Ku-band, the most basic and important television earth station parameter is rain margin. Rain margin is the additional carrier-to-noise power ratio ("C/N") at which a ground receiving station must operate above the C/N that is required for adequate reception under clear sky conditions in order to compensate for the effects of increased attenuation resulting from adverse atmospheric conditions such as rain, sleet and snow. More generally, such margin is referred to as the impulse noise margin. If C/N drops too low, the video picture quality is degraded. Impulses or "sparklies" appear on the screen and at some point the picture is lost entirely. Studies have shown that attenuation caused by rain is the dominant factor to be considered in deciding how much margin is required in ground station design. The larger the margin, the greater the ability of the system to maintain a clear picture despite precipitation caused signal attenuation.
The required rain margin is chosen by users according to their criteria for acceptable signal quality during precipitation and the expected frequency and severity of such precipitation. These decisions are normally based on data published by the International Radio Consultative Committee for various geographical areas of the United States or on similar data for other countries.
In earth station design, C/N can be increased by increasing the antenna gain through improved efficiency and by using larger apertures and amplifiers with lower noise figures. Decreased C/N can be caused by lower satellite Effective Isotropic Radiated Power (EIRP). Antenna related problems which could decrease C/N include antenna equipment malfunction, improper antenna installation, antenna misalignment, polarization misadjustment and receiver detuning.
When an antenna system is installed, normal engineering practice requires that its operational parameters, including its rain margin, be tested to ensure conformance with specifications. A number of techniques have been used in the past to measure a ground station's C/N to determine whether adequate rain margin exists. In a commonly used method involving the use of a power meter attached to the ground station's amplifier output, C/N is measured by taking the ratio of the output during "clear sky" reception to the power output resulting from noise when the carrier is not present. Noise power can be measured by pointing the antenna away from the satellite or by turning off the carrier signal at the transmitting satellite. For more accurate results, the measured noise power is subtracted from the carrier measurement before the ratio is determined. In this case, EQU C/N =10 Log[(C+N)-N]/N
where C+N is the measured power with the carrier present and N is the measured noise power with the carrier absent. One disadvantage of this method is the requirement for extra equipment such as the power meter and a bandpass filter. In addition, there is a disruption in programming. Furthermore, if the carrier signal is removed by redirecting the antenna, additional manpower may be required for assistance, one does not measure the true noise level in the direction of the satellite and it is possible to pick up stray signals from other sources.
Another technique for measuring C/N involves the use of a spectrum analyzer to directly measure the carrier and noise power components of the earth station output. Spectrum analyzers, however, are not generally used with modulated transponders because their narrow reception band cannot capture the entire range of transmitted signals. In many pay television services, unmodulated carriers are only transmitted for testing once a month.
Other methods for determining C/N involve measurements made by pointing the antenna at a known noise source, such as a radio star or a NBS calibrated noise generator and calculations based on measurements of the demodulated baseband video signal to RMS noise ratio.
Still another prior method for measuring the earth station's C/N requires connecting a variable waveguide attenuator between the receiving antenna horn and the ground station amplifier. The total received signal, which includes the carrier signal plus noise (C+N), is first measured during normal programming reception. Then a waveguide attenuator is inserted between the horn and LNA (low noise amplifier) or LNB (low noise block converter) of the ground station. A bandpass filter is also placed between the output of the amplifier and the power meter to restrict the measurement to the range of interest. The received signal is then completely attenuated and the noise is measured. C/N can then be calculated by the formula discussed above.
This attenuation also causes an increase in system noise. System noise affects C/N and another indicator of antenna performance, G/T. G/T is the ratio between antenna gain in decibels and equivalent system noise temperature in degrees Kelvin, and indicates the efficiency (figure-of-merit) of the satellite earth station. Since system noise is directly related to the system temperature, as attenuation is increased, total noise or system temperature also increases, further degrading C/N. The increase in system noise due to attenuation is equal to delta G/T. The amount of attenuation required to cause impulses, plus delta G/T, is the station's impulse noise margin.
The use of a waveguide attenuator is accurate, but the and bandpass filter can cost between $3,000-$5,000. It also requires the dismantling of the antenna system. Furthermore, T.V. reception must be disrupted for long periods of time. In addition, since the attenuator itself is a source of noise, one is not measuring the actual noise of the antenna system itself. Minimization of this extra noise requires careful instrumentation to properly match the input and output impedances of the attenuator to the output and input impedances of the horn and LNA/LNB, respectively. The addition of the attenuator may also require a repositioning of the horn, which could remove it from the focal point of the amplifying dish.
Other problems exist with currently available techniques for setting up a satellite ground station antenna and optimizing its performance. One such aspect of antenna installation is the proper alignment of the antenna in relation to the satellite. Because of the narrow bandwidth of Ku-band antennas, a small pointing error in azimuthal or elevational angle could reduce the received carrier signal by several decibels. This might not affect T.V. reception on a clear day but it does decrease one's rain margin, potentially interfering with reception during precipitation. Proper alignment, referred to as peaking, can be obtained through the measurement of the carrier level indicator on the receiver or the measurement of AGC (Automatic Gain Control) with a voltmeter. A power meter or spectrum analyzer can also be used. These techniques present the same problems as discussed above in relation to determining C/N.
An additional aspect of the antenna that could need to be adjusted for optimum reception is the receiver tuning. Some receivers include a meter for indicating the tuning. Others have an automatic fine tuning control. This could add additional costs to the antenna system, and can still require testing to ensure proper operation.
A further required adjustment of the antenna system is its resolution of signal polarization. Proper alignment with the horizontal and vertical components of the transmitted electromagnetic wave is required for optimum reception. In a typical configuration, the antenna feedhorn is connected to an orthomode transducer which splits th received signal into its horizontal and vertical components. These components are then amplified by distinct LNAs or LNBs. The feedhorn, transducer and LNA/LNB are rigidly connected so that rotation of the feedhorn changes the rotational position of the transducer and LNA/LNB in relation to the polarization of the incoming wave. The proper alignment can be determined with a power meter by monitoring the power level of one of the LNBs amplifying one component of the incoming signal, while rotating the feedhorn. The rotational position yielding the minimum and maximum power level for a horizontally or vertically polarized wave can then be determined. Because the two LNBs are already adjusted to receive signals 90.degree. to apart from one and other, the position yielding a minimum power reading for one component will be the optimum position for the other component. Generally, this minimum, or null, can be more accurately determined than a maximum. The null position of either the horizontal or vertical component of the carrier signal will, therefore, determine the proper orientation of the feedhorn with respect to both.
Some antennas include a resonant element to couple the signal received by the feedhorn to the amplifier. The orientation of this probe can be varied to be in alignment with either the horizontal or vertical component of the signal. The optimum position of the probe, which can be driven by a motor, can be determined with a power meter by determining the position of the probe yielding the maximum power level.